Process for manufacturing stainless steel containing fine carbonitrides, and product obtained from this process

ABSTRACT

The invention relates to a process for manufacturing a semi-finished product made of stabilized stainless steel, which includes a casting step by means of a hollow jet nozzle placed between a tundish and a continuous casting mould. The nozzle comprises, in its upper part, a distributing member for deflecting the liquid metal arriving at the inlet of the nozzle, thus defining an internal volume with no liquid metal in the hollow jet. A non-stabilized stainless steel containing no nitride, carbide and carbonitride precipitates, is delivered in liquid metal form into the tundish. The liquid metal is cast by means of the nozzle, simultaneously carrying out an addition of metal powder into an internal volume of the hollow jet. The metal powder contains at least one element for stabilizing the stainless steel, the addition being carried out at a temperature between T liquidus +10° C. and T liquidus +40° C. The liquid metal is solidified, the solidification starting less than 2 seconds after the addition, in order to obtain the semi-finished product.

The invention relates to a process for manufacturing stabilized stainless steels for inexpensively obtaining a very fine dispersion of carbonitrides after solidification, with a minimized risk of nozzle blockage during casting.

The invention also relates to continuously cast stabilized stainless steels having a very fine dispersion of uniformly distributed carbonitrides. To stabilize these stainless steels, in-ladle additions of stabilizing elements are carried out. Now, it is known that any precipitation of chromium carbide at the grain boundaries may lead to a local depletion of chromium and therefore to a sensitivity to intergranular corrosion. Elements such as titanium, zirconium, niobium and vanadium, which form carbides, nitrides or carbonitrides that are more stable than chromium carbides, are therefore used as stabilizing elements for fixing carbon and nitrogen.

In-ladle additions of titanium or ferro-titanium are for example carried out in flux-cored wire or sponge form. However, there are drawbacks with these early additions, that is to say at the ladle stage:

-   -   because of the time elapsing between the additions and the         in-mould solidification, some of the precipitates have time to         coalesce and agglomerate within the liquid metal, resulting in         an increase in the mean precipitate size and the presence of         certain precipitates of larger size. This has a deleterious         influence on the mechanical properties since damage initiation         takes place firstly on the larger precipitates. Furthermore,         certain precipitate agglomerates may be found on the skin of         semi-finished products after casting and may result in surface         defects that have to be removed by expensive mechanical         treatments;     -   moreover, partial oxidation of the stabilizing elements may         occur and a number of precipitates have the time to separate,         thereby appreciably reducing the effectiveness of the additions         of these elements.

It is envisaged to stabilize stainless steels at the continuous casting stage. The continuous casting of steel is a well-known process. It consists in pouring a liquid metal from a ladle into a tundish intended to regulate the flow and then, after this tundish, in pouring the metal into the upper part of a water-cooled bottomless copper mould undergoing a vertical reciprocating movement. The solidified semi-finished product is extracted from the lower part of the mould by rollers.

The liquid steel is introduced into the mould by means of a tubular duct called a nozzle placed between the tundish and the mould.

Thus, a pouring device is proposed for additions in the mould stage, as described in the Centre de Recherches Métallurgiques patent EP 269 180. The liquid metal is poured onto the top of a dome made of a refractory material of a tundish member. The shape of this dome causes the metal to flow towards its periphery, the flow being deflected towards the internal wall of the nozzle or of an intermediate vertical tubular member. What is thus created, in the central part of the nozzle beneath the tundish member, is a volume without any liquid metal within which it is possible to carry out additions via an injection channel. The device thus described is referred to as a hollow jet nozzle.

Using this device, patent BE 1 014 063 describes a method of adding metal powders in order to form oxides during solidification. For this purpose, a steel having a given amount of dissolved oxygen (O₂) is poured from the tundish into the mould, an addition (M) of metal powder is carried out, the M/O₂ ratio is checked and the powder is mixed with the liquid metal so as to form metal oxides.

Even though the formation of these oxides may play a favourable role by increasing the equiaxed zone fraction on the solidified semi-finished product, this method does not however help to stabilize stainless steels since it does not involve trapping the carbon and nitrogen. Moreover, application of such a method to stainless steels is not mentioned in the above patent.

Patent WO 2006/096942 describes an addition of high-performance ceramic nanoparticles within a hollow jet nozzle. These ceramic nanoparticles may be oxides, nitrides, carbides, borides or silicides and are characterized by a high thermal stability, so that practically no reaction occurs between them and the liquid metal. However, this method is difficult to implement due to agglomeration of the nanoparticles, which tend to form larger particles possibly causing the abovementioned defects. Here again, the application of such a technique to stainless steels is not mentioned in the above patent.

The object of the invention is to provide a process for manufacturing stabilized stainless steels having a fine uniform dispersion of nitrides and/or carbonitrides. The aim in particular is to obtain a large number of fine precipitates, with a size of less than 2.5 microns, while still limiting the number of coarse precipitates of size greater than 10 microns.

Another object of the invention is to provide a process which is more to efficacious as regards the efficiency of the additions of stabilizing elements, compared with in-ladle addition processes.

Another object of the invention is to provide a process for minimizing the risk of nozzle blockage when continuously casting stainless steels.

Another object of the invention is to provide stainless steel semi-finished products of equiaxed solidification structure after the continuous casting, even without employing electromagnetic stirring techniques.

Another object of the invention is to provide stainless steel semi-finished products which are very uniform over a cross section in relation to the continuous casting direction.

Thus, the subject of the invention is a process for manufacturing a semi-finished product made of stabilized stainless steel, which includes a casting step by means of a hollow jet nozzle placed between a tundish and a continuous casting mould, said nozzle comprising, in its upper part, a distributing member for deflecting the liquid metal arriving at the inlet of the nozzle, thus defining an internal volume with no liquid metal. The process is characterized in that a non-stabilized stainless steel containing no nitride, carbide and carbonitride precipitates, is delivered in liquid metal form into the tundish; then the liquid metal is cast by means of the nozzle, simultaneously carrying out an addition of metal powder into the internal volume of the hollow jet, the metal powder containing at least one element for stabilizing the stainless steel, the addition being carried out at a liquid steel temperature between T_(liquidus)+10° C. and T_(liquidus)+40° C.; and the liquid metal is solidified, the solidification starting less than 2 seconds after the addition, in order to obtain the semi-finished product.

The subject of the invention is also a process according to one of the above embodiments, characterized in that the stabilizing element is chosen from one or more of the following elements: titanium, niobium, zirconium and vanadium.

Preferably, the stabilizing element is titanium, the titanium, carbon and nitrogen contents of the stainless steel satisfying, the contents being expressed in percentages by weight: Ti≧0.15+4(C+N).

According to one particular embodiment, the steel is a ferritic stainless steel or an austenitic stainless steel or a martensitic stainless steel or an austeno-ferritic stainless steel.

The subject of the invention is also a semi-finished product manufactured by a process according to one of the above embodiments, characterized in that it has a fully-equiaxed solidification structure.

The subject of the invention is also a stainless steel product manufactured from a semi-finished product produced by a process according to one of the above embodiments, characterized in that the stabilizing element is titanium and in that the number of titanium nitride or carbonitride precipitates of size smaller than 2.5 microns is greater than 15 000/cm².

The number of titanium nitrides or carbonitrides of size greater than 10 microns is preferably less than 50/cm².

According to a preferred embodiment, the mean inter-precipitate distance is less than 15 microns.

Other features and advantages of the invention will become apparent over the course of the description below, given by way of example and with reference to the appended FIG. 1, which shows schematically an example of a device for implementing the process according to the invention.

Other features and advantages of the invention will become apparent over the course of the description below, given by way of example.

The invention presented relates to a wide range of stainless steels capable of being stabilized by additions of titanium, niobium, zirconium, vanadium or other stabilizing elements, these elements being used separately or in combination.

In particular, the invention may be advantageously implemented in the manufacture of ferritic stainless steels of the X 3CrTi17 type, with the following composition according to standard NF.EN 10.088-1 and 2: C<0.050%, Si<1.00%, Mn<1.00%, P<0.040%, S<0.015%, Cr:16.00-18.00%, N<0.045%, 0.15+4(C+N)<Ti<0.080%, the contents being expressed in percentages by weight.

The process according to the invention is the following:

-   -   a liquid metal intended for the manufacture of ferritic         stainless steel, austenitic stainless steel, martensitic         stainless steel or austeno-ferritic stainless steel is produced         by means of a process known per se. At the ladle stage, before         pouring, the liquid steel may be subjected to various         metallurgical operations:     -   complementary additions for grading the steel;     -   deoxidation of the liquid metal;     -   stirring of the bath by an inert gas so as to ensure thermal         homogenization before pouring.

At this stage, even though the liquid metal may possibly contain a small amount of an element for stabilizing the stainless steel, no precipitation of this element occurs. The principal addition of stabilizing element and its precipitation take place subsequently, as described below.

A liquid metal containing nitrogen N and carbon C, which are present in the form of dissolved elements, is poured from the ladle into the tundish. The composition and the temperature of the liquid metal are such that no nitride, carbide or carbonitride precipitates exist under these conditions. The carbon and nitrogen contents are used to adjust the amounts of stabilizing elements that will be added subsequently.

The content of the ladle is poured into a tundish 1 having a bottom with a closure device 2, the degree of sealing of which enables the flow into a pouring nozzle 3 to be regulated. At this stage, the temperature of the liquid steel must not be too high. This is because, as will be seen later, the additions within the hollow jet nozzle must be carried out at a temperature not too far above the liquidus temperature (denoted by T_(liquidus)) of the steel.

A person skilled in the art will know, by means of his general knowledge and the specific features of the casting device that determine the temperature drop between the tundish and the nozzle, how to adjust the pouring temperature according to features of the invention explained below.

As explained, the process according to the invention requires the use of a hollow jet nozzle. This nozzle has a distributing dome 4 made of a refractory material pierced by one or more injection channels that open into the central lower part of the dome in the form of injection tubes 5. It is thus possible to add a metal powder conveyed by a carrier gas. The injected powder 6 mixes with the liquid metal which is deflected by the upper part of the dome towards the walls of the nozzle or of an intermediate tubular member between the actual nozzle and the tundish.

The powder is fed by one or more tubes 7 which are themselves connected to one or more reservoirs 8. The upper part 9 of these powder reservoirs is under pressure from an inert carrier gas such as argon, enabling the powder to be protected from oxidation. A suitable flow of gas forces the powder to flow into the hollow jet nozzle with a flow rate corresponding to the desired amount to be added. The flow of powder may also be facilitated by a mechanical device, such as a feed screw. The particle size of the powder must be chosen so as to ensure that there is easy flow between the reservoirs and the nozzle and almost immediate melting in the liquid metal. A spherical particle size between 100 and 200 microns is very suitable for these requirements.

This powder contains one or more metal elements intended to stabilize the stainless steel, namely:

-   -   titanium, which may be used pure or in ferro-titanium form for         cost reasons, these additions being intended to form titanium         nitrides TiN which are very stable or carbonitrides Ti(C,N);     -   zirconium, which also forms very stable nitrides and         carbonitrides;     -   niobium, which is essentially intended to form carbonitrides         Nb(C,N);     -   vanadium, which also forms carbonitrides.

Powders of these metal elements may of course be mixed together so as to produce a particular combination, such as for example a titanium-niobium two-stabilizer blend. It is also possible to blend the above powders with ferro-alloys or with iron powder for the purpose of reducing the superheating temperature at the outlet of the hollow jet nozzle so as to increase the equiaxed zone fraction of the semi-finished product after solidification.

At the same time as the pouring, the powder comprising the stabilizing element or elements is added to a liquid metal at a temperature between T_(liquidus)+10° C. and T_(liquidus)+40° C. This particular addition temperature range makes it possible simultaneously:

-   -   to obtain an intense precipitation of fine nitrides and         carbonitrides; and     -   to promote solidification in equiaxed form.

When the addition temperature is too high relative to the liquidus, the time that elapses between the formation of nitrides or carbonitrides and the end of solidification increases, thereby resulting in an increase in their size, this being an undesirable phenomenon.

However, when the addition temperature is too low relative to the liquidus, the process becomes more sensitive to an inopportune variation in the manufacturing parameters, with a risk of blocking the nozzle.

As soon as the addition into the hollow jet nozzle has taken place, the stabilizing element is melted by contact with the liquid metal in a few tenths of a second. Since the powder is protected from oxidation by the inert gas until its contact with the liquid metal, the effectiveness of the addition is high.

A sufficient amount of stabilizing elements are added in order for the nitrogen and carbon to be fully precipitated and for the solubility product corresponding to the formation of these precipitates to reach or exceed the temperature at which the addition is carried out. The nitrides and/or carbonitrides then precipitate immediately in a very fine form.

After addition, the liquid metal starts to solidify in less than 2 seconds, solidification starting on the walls of the mould 10. This very short time in which the precipitates are maintained in the liquid metal prevents them from increasing in size. A person skilled in the art will know how to adapt the various parameters at his disposal, such as the following: height of the injection device relative to the mould; injection rate; greater or lesser power of the heat exchangers; semi-finished product extraction rate; superheating temperature; complementary injection of ferro-alloy powder in order to speed up the solidification, in order for the time between addition and start of solidification to be less than 2 seconds.

A preferred embodiment is based on the use of titanium for the purpose of precipitating fine dispersed nitrides and/or carbonitrides. According to the invention, the titanium, carbon and nitrogen contents of the stainless steel, expressed in percentages by weight, are such that: Ti≧0.15+4(C+N). Under these conditions, the amount of titanium added enables the steel to be fully stabilized.

One particular feature of the stainless steels obtained according to the invention lies in the great uniformity in the dispersion of the nitrides and carbonitrides with a shorter mean inter-precipitate distance, so that any sensitivity because of a locally depleted zone is reduced.

According to another preferred embodiment of the invention, the above parameters, and especially the powder injection rate and the superheating temperature, are adapted so as to obtain a semi-finished product with a fully equiaxed solidification structure. The term “semi-finished product” denotes for example a slab (with a thickness of around 200 mm), a thin slab (with a thickness of around 50-80 mm), a thin strip (with a thickness of around 1-3 mm) or a billet, which is not yet mechanically hot-deformed. Such an equiaxed structure is particularly advantageous in the field of ferritic stainless steels in order to minimize roping defects. It is known that such defects are manifested by the formation of surface irregularities after drawing that are parallel to the rolling direction. They are due to the presence of heterogeneous structures before cold rolling and annealing, which structures themselves result from columnar solidification structures.

The powder addition proves to be advantageous for obtaining a fully equiaxed structure since the precipitates act as nucleation sites, thus preventing less favourable columnar or basaltic solidification. The invention therefore optionally makes it possible to employ electromagnetic stirring techniques that are normally used for this purpose.

After the semi-finished product has been manufactured, it may be hot-rolled or cold-rolled, pickled and annealed, according to conventional processes, in order to obtain as it were a product that can take various forms, such as a hot-rolled strip, thin sheet or long product of various shapes.

In the absence of a solutioning treatment, the precipitation characteristics are practically identical on the semi-finished products and the products obtained from these semi-finished products. The advantages conferred by the invention on the semi-finished products are therefore also on the products obtained.

As a non-limiting example, the following results demonstrate the advantageous characteristics conferred by the invention.

EXAMPLE

Two titanium-stabilized ferritic stainless steel heats, the compositions of which, expressed in percentages by weight, are given in Table 1 were produced. Steel A was produced according to the invention under conditions that will be explained, while steel B was manufactured using a conventional continuous casting technique.

TABLE 1 Composition of the steels C Mn Si Cr Cu Ni S Ti V N A 0.016 0.34 0.38 16.27 0.05 0.10 0.006 0.30 0.12 0.015 B 0.02 0.34 0.38 16.16 0.04 0.16 0.006 0.45 0.08 0.012 A = Manufactured according to the invention B = Manufactured according to a conventional technique

In grade B, titanium was added in the form of titanium sponge to the ladle. When producing grade A according to the invention, the liquid metal in the tundish contained no titanium. This element was added within a hollow jet nozzle in the form of a ferro-titanium powder (30% iron/70% titanium) with a particle size between 100 and 200 microns. The powder addition temperature was T_(liquidus)+35° C. The metal started to solidify on the walls of the mould less than two seconds after addition. Various heats were formed into slabs according to the invention without encountering any nozzle blockage problem.

This is a consequence of the tardy precipitation characteristic of the process, of the short time in which the precipitates are held within the liquid metal and an advantage over conventional methods of addition.

After the slabs were hot-rolled in order to obtain strips 3 mm in thickness, the presence of titanium nitride precipitates was revealed on polished sections. The size distribution of these precipitates was measured by image analysis using the procedure defined in the ASTM E1245 standard. The precipitate density is expressed as the number of precipitates per cm².

The mean inter-precipitate distance was also measured. The results of these to measurements are given below:

TABLE 2 Precipitate distribution characteristics Number of TiN Number of TiN particles with a particles with a size Mean inter- size smaller than greater than 10 μm precipitate distance 2.5 μm (N/cm²) (N/cm²) (microns) Steel A 17560  30 14.2 (invention) Steel B  9320 110 24.6 (reference) Underlined values: not according to the invention

A fine precipitate (<2.5 μm) density greater than 15 000/cm² guarantees that the titanium nitrides are very uniformly distributed. Therefore, the carbon and nitrogen are very completely and uniformly trapped.

A coarse precipitate (>10 μm) density less than 50/cm² ensures that fracture initiation does not take place prematurely during mechanical stressing.

These two characteristics are observed in the case of the steel manufactured according to the process of the invention. Compared with a conventional process, the invention makes it possible to increase the number of fine precipitates by a factor of about 2 and to reduce the number of coarse precipitates by a factor of about 3.

Observations were made on a section transverse to the casting direction on a strip 1 m in width and 3 mm in thickness manufactured according to the invention. The measurements carried out at the centre, at ⅓ width, ⅔ width and at the edge of the strip showed that the precipitation was very uniform. In particular, the mean inter-precipitate distance was practically the same between the centre and the edge of the strip. The semi-finished products and the products manufactured according to the invention are therefore very uniform in terms of structure and properties.

In addition, the solidification structure examined on polished and etched cross to sections of slabs was fully equiaxed. The absence of columnar zones proves to be favourable for preventing roping defects.

The efficiency of titanium addition (the ratio of titanium present in the final product to titanium added in powder form) is 95 to 100% in the process according to the invention. This efficiency is therefore very much higher than that of the conventional process, which is around 60%.

The process according to the invention therefore makes it possible for stabilized stainless steel grades having a very fine dispersion of nitrides or carbonitrides to be manufactured inexpensively and reliably. 

1. Process for manufacturing a semi-finished product made of stabilized stainless steel, which includes a casting step by means of a hollow jet nozzle placed between a tundish (1) and a continuous casting mould (10), said nozzle comprising, in its upper part, a distributing member (4) for deflecting the liquid metal arriving at the inlet of said nozzle, thus defining an internal volume with no liquid metal, characterized in that: a non-stabilized stainless steel containing no nitride, carbide and carbonitride precipitates, is delivered in liquid metal form into said tundish; then said liquid metal is cast by means of said nozzle, simultaneously carrying out an addition of metal powder (6) into said internal volume of said hollow jet, said metal powder containing at least one element for stabilizing said stainless steel, said addition being carried out at a liquid steel temperature between T_(liquidus)+10° C. and T_(liquidus)+40° C.; and said liquid metal is solidified, the solidification of said liquid metal starting less than 2 seconds after said addition, in order to obtain said semi-finished product.
 2. Process according to claim 1, characterized in that said stabilizing element is chosen from one or more of the following elements: titanium, niobium, zirconium and vanadium.
 3. Process according to claim 2, characterized in that said stabilizing element is titanium, the titanium, carbon and nitrogen contents of said stainless steel satisfying, the contents being expressed in percentages by weight: Ti≧0.15+4(C+N).
 4. Process according to any one of claims 1 to 3, characterized in that said steel is a ferritic stainless steel or an austenitic stainless steel or a martensitic stainless steel or an austeno-ferritic stainless steel.
 5. Semi-finished product manufactured by a process according to any one of claims 1 to 4, characterized in that it has a fully-equiaxed solidification structure.
 6. Stainless steel product manufactured from a semi-finished product produced by a process according to any one of claims 1 to 4, characterized in that the stabilizing element is titanium and in that the number of titanium nitrides or carbonitrides of size smaller than 2.5 microns is greater than 15 000/cm².
 7. Product according to claim 6, characterized in that the number of titanium nitrides or carbonitrides of size greater than 10 microns is less than 50/cm².
 8. Product according to claim 6 or 7, characterized in that the mean inter-precipitate distance is less than 15 microns. 